Chem 323
B. Terem
SYNTHESIS OF DIPHENYLACETYLENE
Br
NaOH
Br
meso-1,2-dibromo-1,2-diphenylethane
HO
O
OH
diethylene glycol
m.p. 242oC
165oC
diphenylacetylene
m.p.61oC
A convenient method for the preparation of alkynes involves a base-promoted double
dehydrohalogenation of a vicinal dihalide1. In the reaction above two moles of HBr are
eliminated from meso-1,2-dibromo-1,2-diphenylethane (meso-stilbene dibromide) successively.
Elimination of the second mole of HBr is more difficult than the first and requires strong heating.
For this purpose, a high boiling, water miscible solvent such as diethylene glycol
(2,2'-oxydiethanol) is useful.
The procedure below is given for 1.0 g of meso-1,2-dibromo-1,2-diphenylethane. You should
save 0.05 g of meso-1,2-dibromo-1,2-diphenylethane you have prepared earlier and use the rest of
the material for the synthesis of diphenylacetylene by scaling the amounts accordingly.
PROCEDURE:
In a 20 X 150 mm test tube place 1.0 g of meso-1,2-dibromo-1,2-diphenylethane, 6 pellets of
sodium hydroxide (500 mg), and 4 mL of diethylene glycol. Insert a thermometer into a
10 X 75 mm test tube containing enough diethylene glycol to cover the bulb, and slip this
assembly into the larger tube. Clamp the tube in a vertical position in a hot sand bath, and heat the
mixture to a temperature of 160oC. By intermittent heating, keep the mixture at 160o-170o for a
period of 5 minutes, then allow the mixture to cool down to room temperature. When the mixture
is cooled down sufficiently, remove the thermometer and the smaller test tube, and add 10 mL of
water. Collect the diphenyl acetylene, which will have separated as a nearly colorless granular
solid, by suction filtration. The crude product need not be dried, but can be crystallized directly
from 95% ethanol. Let the solution stand undisturbed in order to observe the formation of
beautiful, very large spears of colorless crystals.
REFERENCES (to Smıth, Organic Chemistry)
1
Sect. 8.10 pp. 301-302